It is known in the art how to generate sinusoidal signals using inductors and capacitors as the resonant elements. FIG. 1 shows the sinusoidal signal that results from the LC tank circuit of FIG. 2. In FIG. 2, inductor 210 (L) is connected in parallel with capacitor 220 (C) and the output voltage 250 (Vout) is measured across the two components. Although such sinusoidal signals can be generated with high efficiency, i.e., minimal power loss, these signals have many limitations when used for digital and analog applications. In the case of digital logic, there is no well defined “high” or “logical one” voltage level and likewise for the “low” or “logical zero” voltage level. Furthermore, the slow rise and fall times of the sinusoidal signal increases the response time of the interfacing circuits and is an additional source of power dissipation in the interfacing circuits.
These limitations of a sinusoidal signal are partially overcome by replacing the lower portion of the sine wave with a substantially flat section, as illustrated in FIG. 3. Such a technique is described in U.S. Pat. No. 5,559,478 and in the article by W. Athas, et al., entitled “A Resonant Signal Driver for Two-Phase, Almost Non-Overlapping Clocks,” published in the Proceedings of the 1996 International Symposium on Circuits and Systems, 1996. The occurrence of a sinusoidal pulse defines the high or logical one level and the absence the low or logical zero level, or vice versa.
However, the waveform of FIG. 3 is not suitable for many applications because it is highly desirable that the binary values be defined as levels with short transition times between the levels. The desired waveform is the trapezoidal signal of FIG. 4 in which the top and bottom are flat, and the rise and fall times are fast while minimizing power dissipation. Unfortunately, by conventional means the power required to generate the waveform of FIG. 4 for a capacitive signal load is at least twice the signal power. It is therefore desirable to generate a waveform similar to that shown in FIG. 4 while minimizing the power requirements.
Note that for the application domain of power electronics, it is often desirable to use a power MOSFET as a switch to control a large current. The power dissipation due to the on-resistance of the power MOSFET equals the square of the current times the on-resistance of the MOSFET, as illustrated below:Pon=I2Ron  (1)
The larger the gate area of the MOSFET, the lower the value of the on-resistance. However, the input gate capacitance increases with the size of the MOSFET. If the input gate is driven from a non-resonant circuit, the power dissipation of the gate is at least:Pgate=FCgateV2  (2)
where F is the frequency at which the gate is pulsed and V is the voltage potential between the gate and ground. To a first approximation the on-resistance (Ron) and gate capacitance (Cgate) are inversely proportional. Total power is minimized when the power required to drive the gate equals the power dissipation due to the on-resistance of the power MOSFET.
Another source of power dissipation is the interval during which the gate voltage swings between its minimum value and maximum value. During such intervals the on-resistance must change from a very high resistance (on the order of megaohms) to a very small resistance (on the order of milliohms). The power dissipation during these intervals depends on the time duration of the interval and the shape of the waveform during the interval. Resonant circuits for powering the gate will dissipate less energy than a non-resonant circuit, but the long duration of the transitions will introduce larger power dissipation in the on-resistance of the power MOSFET.
Thus, there continues to be a very substantial need for waveform generating systems driving loads having significant capacitive reactance which have voltage transitions that are substantially shorter in time duration than the sinusoidal waveform, which have substantially flat top and bottom sections, and which minimize power dissipation below the conventional FCV2 limits.